1. Field of the Invention
The present invention relates to a magnetic transfer system for transmitting a driving force to a movable body by a non-contact system, suitable for a driving mechanism for transferring an object in a vacuum environment isolated from the atmosphere, and capable of performing smooth transfer generating less dust, a power transmission mechanism of the magnetic transfer system, and a rotational driving member preferably used for the magnetic transfer system.
2. Description of the Related Art
To transfer an object in a vacuum environment, a rack and pinion mechanism, roller-type driving mechanism, and chain driving mechanism have been frequently used. These driving mechanisms comprise a system for directly transmitting a driving force by a contact action and are referred to as a contact transmission system. This type of driving mechanism has the poor characteristics that friction coefficient increases and lubricating oil cannot be used in the vacuum environment. Therefore, there are problems that abrasion loss extremely increases and much dust is generated (this state is referred to as "dust generation"). Moreover, because the friction coefficient increases, the clearance of a contact portion must be increased and thereby an accurate movement is interrupted.
Furthermore, recently as represented by a semiconductor, it has been requested to extremely limit an amount of dust to be attached to an electronic part or the like. A driving mechanism capable of completely preventing dust from being generated is ideally desired.
To decrease the amount of dust, a transfer system of the non-contact transmission type is clearly preferable. Various systems have been proposed so far as the transfer system of the non-contact transmission type. Among the proposed systems, a system using the action of electromagnetic coupling (hereafter referred to as "a magnetic transfer system") has a relatively simple structure. A linear transfer mechanism constituted by combining a spiral magnetic circuit with an magnetic electrode has been recently proposed as the magnetic transfer system (official gazette of U.S. Pat. No. 5,377,816). Moreover, a magnetic screw used as a feeder is proposed as an art indirectly related to the magnetic transfer system in the field of machining tools or the like (official gazette of Japanese Patent Application Laid-Open No. 7-280060).
The above official gazette of U.S. Pat. No. 5,377,816 discloses a mechanism for generating a linear motion by using a spiral magnet. That is, the mechanism comprises a cylindrical body to be rotated by a motor, on which many magnet pieces of N and S poles are alternately arranged so as to form a spiral layout and a base member movably arranged closely to the cylindrical body in its axial direction and provided with a magnet segment, and which is constituted so that the base member linearly moves by magnetic attraction the magnet pieces and the magnet segment when the cylindrical body rotates.
Moreover, the above official gazette of Japanese patent Laid-Open No. 7-280060 discloses a magnetic screw. The magnetic screw comprises a screw shaft which is made of a magnetic material and on whose surface a thread is spirally formed and a cylindrical nut body arranged on the outer periphery of the screw shaft at an interval and provided with a spirally-polarized magnetic pole on the inner periphery correspondingly to the thread. When the screw shaft rotates, the cylindrical nut body moves in the axial direction of the screw shaft.
According to the above-described general magnetic transfer system, a contact portion related to a transfer mechanism in a vacuum device is usually only a roller section for supporting a movable body (a carrier or tray; hereafter referred to as "carrier") and a contact portion is absent at the driving-force transmission section for moving the carrier. Therefore, it is possible to eliminate the contact portion causing dust to be generated. Therefore, the magnetic transfer system is able to moderate the problems of abrasion and dust generation compared to the contact transmission system and has an ideal structure as the internal transfer mechanism of the vacuum device.
Because the above-described magnetic transfer system generally has only a small thrust which is obtained from the magnetic coupling action of a magnetic coupling section, it is requested to form many magnetic coupling sections. In this case, the "magnetic coupling section" denotes a section in which magnetic poles are coupled with each other because a magnetic force works a driving section and a driven section. Therefore, in the case of a magnetic transfer system comprising a columnar rotational driving section with a spiral magnet portion formed on its surface (hereafter referred to as "driving shaft") and a movable body provided with magnets arranged at an interval of the same distance as the pitch of the spiral magnet portion, the number of magnetic coupling sections is increased by increasing the length of the driving shaft in the transfer direction (axial direction) and increasing the number of turns of the spiral magnet portion and the number of magnets of the movable body.
For example, a case is considered in which the above magnetic transfer system is applied to a semiconductor fabrication equipment including a plurality of vacuum process chambers connected in series. The chambers are separated from each other by a gate valve. Therefore, when using the magnetic transfer system as a system for transferring the carrier for mounting a substrate, it is a matter of course that the above driving shaft is provided for each chamber so as to move the carrier by rotating each driving shaft. The carrier is transferred through each chamber in order in accordance with a predetermined substrate processing procedure. In this case, the carrier is constituted so that it is delivered the driving shafts of the chambers while it is moved the adjacent chambers.
The conventional magnetic transfer system having the above structure has the following problems.
Because the chambers are separated from each other by the gate valve, a gap is present the driving shafts and therefore, a crossing portion is formed for the carrier to be transferred. Therefore, to smoothly move the carrier, it is necessary to match magnetic circuits driving shafts at the crossing portion. Unless properly performing the matching, the carrier cannot be smoothly moved. Moreover, if repulsion occurs in carrier delivery at the crossing portion, not only a purposed thrust is not produced but also transfer may stop.
Therefore, to smoothly move the carrier driving shafts, it is desired to synchronize the rotations of the driving shafts. As a simple structure for realizing the synchronization, it is considered to rotate all the driving shafts of each chamber by a motor. However, because this structure has a large problem that the load excessively increases, it cannot be practically used. Then, it is considered to perform the control for synchronization the driving shafts by providing motors which rotate the driving shafts for each chamber and synchronously controlling the operation of each motor. In the case of this synchronous control, however, it is practically very difficult to align the positions of a plurality of magnetic poles arranged on the carrier with the position of the spiral magnet portion of the driving shaft when delivering the carrier the chambers because the stop position of the object to be processed is independently set for each chamber and dimensional errors of the chambers or errors in assembling the semiconductor fabrication equipment are present.
As described above, in the case of the conventional semiconductor fabrication equipment provided with a plurality of vaccum process chambers connected in series and providing a driving shaft for each chamber so that it rotates in an independent driving system, it is difficult to perform the control for aligning the position of the magnetic pole arranged to the carrier with the position of the next driving shaft when delivering the carrier from the driving shaft of each chamber to the driving shaft of adjacent other chamber.
The above problems are not included in the above-described documents of prior art. Therefore, the problems cannot be solved by the magnetic feed mechanism or magnetic screw shown in these documents.